In conventional oil drilling operations, the drill stem is made up of a plurality of sections which are joined together as the drill stem is lowered into the hole and which can be disconnected as the drill stem is raised from the hole. When replacing a drill bit, for example, it is necessary to withdraw the drill stem from the hole, disconnecting and storing the drill stem sections one at a time. The lowering of the drill stem into the hole or raising it from the hole is interrupted periodically to permit each section to be connected or disconnected from the string.
Automatic mechanized drilling rigs have been heretofore proposed which permit the continuous raising and lowering of the drill stem, each section being connected and disconnected without interruption of the vertical movement of the pipe string. Such an arrangement is described, for example, in U.S. Pat. No. 3,002,560.